Thyroid hormone [3,5,3'-triiodothyronine (T.sub.3)] exerts diverse metabolic effects on mammalian cells, primarily through its interaction with nuclear receptor proteins of the cells. The T.sub.3 -receptor complex regulates expression of nearby target genes by transcriptional and posttranscriptional mechanisms.
Nuclear receptors for thyroid hormone have been identified as products of the c-erbA protooncogene. (Weinberger, C. et al., Nature 324:641 (1986); Sap, J. et al., Nature 324:635 (1986)). The c-erbA gene is the cellular counterpart of the viral oncogene v-erbA. Sap et al. (Nature 324:635-640 (1986)) reported that the c-erbA protein is a nuclear protein which functions as a receptor for thyroid hormone, whereas the v-erbA product is located in the nucleus but is defective in binding thyroid hormone. In the rat, three c-erbA-related cDNAs have been described and designated r-erbA.alpha.-1, r-erbA.alpha.-2 and r-erbA.beta.-1. The proteins encoded by these genes are classified as alpha or beta forms on the basis of the predicted amino acid sequences and have homologs in man.
The rat r-erbA.alpha.-1and r-erbA.alpha.-2 mRNAs represent alternative splice products of a single rat erbA.alpha. gene. This gene encodes proteins that are identical for the first 370 amino acids from the N-terminus, and then diverge completely.
The protein encoded by erbA.alpha.-1 binds thyroid hormone, whereas the protein encoded by erbA.alpha.-2 does not. The thyroid hormone binding domain is included within the divergent regions of the two proteins, and this may account for the difference in binding between the two proteins. (Lazar, M. A. et al., Mol. Endocrinol. 2:893 (1988); Izumo, S. et al., Nature 334:539 (1988); Mitsuhashi, T. et al., Proc. Nat. Acad. Sci. 85:5804 (1988)).
In the rat, expression of the erbA genes appears to be organ-specific. The erbA.alpha.-1 mRNA is most abundant in skeletal muscle and brown fat, whereas erbA.alpha.-2 is most highly expressed in brain and hypothalamus. The related erbA.beta.-1 mRNA is highly expressed in kidney and liver. (Koenig, R. J. et al., Proc. Nat. Acad. Sci. 85:5031 (1988)). The protein product of erbA.beta.-1 mRNA binds thyroid hormone with high affinity.
T.sub.3 is an iodine-containing amino acid secreted by the thyroid gland. Thyroid function in turn is regulated by the thyroid-stimulating hormone (TSH) of the anterior pituitary gland, and the rate of TSH secretion by the pituitary is regulated in part by circulating thyroid hormone levels via feedback inhibition.
Thyroid hormone exerts physiological effects throughout the body. Thyroid hormone increases oxygen consumption in almost all metabolically active tissues, with a concomitant increase in nitrogen excretion. If food intake does not compensate for the loss of nitrogen, endogenous protein and fat stores are catabolized and weight is lost. The circulating levels of thyroid hormone are important in regulating physiological activity throughout the body, and inappropriately high or low levels of thyroid hormone are implicated in metabolic disorders.
Hypothyroidism, characterized by low circulating levels of thyroid hormones, is associated with a variety of physiological abnormalities. In children, bone growth is slowed and epiphyseal closure is delayed. In adults, protein catabolism results in muscle weakness and osteoporosis; there are also secondary effects on carbohydrate and cholesterol metabolism, as well as on the nervous system and the skin.
Hyperthyroidism, characterized by high circulating levels of thyroid hormone, is associated with nervousness, weight loss, heat intolerance, and a high basal metabolic rate. In general, the secretion of TSH from the pituitary gland is depressed in hyperthyroidism because of the negative feedback effect of the high circulating thyroxine and triiodothyronine levels.
In a few patients hyperthyroidism has been associated with pituitary resistance to thyroid hormone. (Spanheimer, R. G. et al., Arch. Intern. Med. 142:1283-1286 (1982)). This condition is characterized by elevated levels of thyroid hormone in association with an inappropriately elevated serum TSH level. The pituitary gland plays a unique role in modulating the circulating levels of thyroid hormones through its secretion of TSH.
The ability to regulate TSH production by the pituitary has broad implications for treating conditions and diseases in which the levels of thyroid hormones are important. As TSH production is regulated in part by means of thyroid hormone, the mechanism of thyroid hormone binding in the pituitary is of great importance for controlling pituitary TSH production.